IT ALL STARTED NEARLY A CENTURY AGO: The genesis of what would eventually become the Central Artery dates back to 1909, when a city commission recommended building a continuous rail tunnel linking North Station and South Station, a link that remains elusive nearly a century later. A 100-foot-wide roadway was to be built on top of the rail tunnel. When city and state officials could not reach consensus on the rail-and-road link through downtown Boston, the plan was shelved.

The first plans for an elevated roadway were developed by the city of Boston in 1930. Robert Whitten, a consultant for the city planning board, proposed among other improvements an elevated north-south highway running through downtown Boston. Working together with the city's traffic department and planning boards, Whitten developed a detailed traffic analysis and forecast that served as a quantitative basis for the proposed highway.

Even with the WPA largess of the Great Depression, the Central Artery project did not receive Federal funding. The onset of World War II further delayed the project amid manpower and materiel shortages.

MEETING THE NEEDS OF POSTWAR TRAFFIC: The Central Artery plan gained serious traction in the immediate postwar years with the commonwealth's 1948 release of the Master Highway Plan for the Boston Metropolitan Area. It made the following recommendation for the artery's route:

The recommended route for the Central Artery crosses the Charles River just above the present Warren Bridge. The traffic on the expressway at this point will be that from the Northwest and Northeast expressways. The Central Artery will not have sufficient capacity, however, to permit traffic from the Northern Expressway to use the Central Artery for downtown distribution.

The route of the elevated highway will cross Washington Street and Haymarket Square, follow Cross Street past the portal of the Sumner Tunnel, and swing into an alignment parallel to Atlantic Avenue. It will pass Northern Avenue, then absorb the block between Purchase Street and Atlantic Avenue as far as Dewey Square. The route will thread between the larger buildings of the Leather District beyond South Station, cross the northwest corner of the Boston Terminal Company yards, and thence continue south in the block between Hudson and Albany streets.

The route will cross to the east side of Albany Street at Troy Street, and continue parallel to Albany Street to the vicinity of Massachusetts Avenue where it will join the Belt Route (Inner Belt Expressway). The Central Artery will provide connections, via the Belt Route, with existing surface arteries and with the expressway routes.

Early expressway planning called for placing the maximum amount of ramps to and from surface streets to improve circulation. The original plan called for the northbound lanes to have 12 on- and off-ramps, while the southbound lanes were to have 13 on- and off-ramps:

The most difficult problem on such a facility is to provide ramps sufficient in number and in the proper locations. Particularly in Boston, it is necessary that traffic to and from the Central Artery be well distributed over all available downtown streets. Even so, these streets will not be equal to the task, and therefore it will be necessary to make several important street widenings and other major changes.

This need for maximum collection and dispersion potential is one factor making it infeasible to use any route for a downtown expressway along the edge of the area, such as an alignment over the present piers extending into the harbor from Atlantic Avenue.

The following geometric design standards were adopted for the Central Artery:

General: conform to "Interstate" standards of the American Association of State Highway Officials (AASHO).

Pavement widths: 40-foot-width on expressways that provided for three 12-foot-wide lanes plus a two-foot-wide offset to the curb; 28-foot-wide ramps that provided for two 12-foot-wide lanes plus a two-foot-wide offset to the curb.

Vertical clearance: 14 feet, 3 inches.

Lateral clearance: six feet from right travel lane, four feet from left travel lane.

Shoulders were not provided on the elevated expressway because of the high costs associated with additional right-of-way acquisition and viaduct construction. It was thought that the frequent placement of on- and off-ramps would obviate the need for shoulders.

The overall Master Highway Plan called for the construction of 87.4 miles of new expressways to be built within the Route 128 boundary at a total cost of $322 million. As the cost of the expressways was too heavy to be borne by the city of Boston and surrounding municipalities alone, the state authorized $100 million in bond financing in 1949, followed by three additional bond issues totaling $450 million. The bonds were to be financed through the state's dedicated highway fund, and not by tolls.

This artist's conception dating back to 1930 shows the first visual representation of an elevated highway above the streets of Boston. The elevated Central Artery had as its genesis a long-abandoned plan for an elevated rail connection between North Station and South Station. (Drawing from Massachusetts State Transportation Library Archives.)

NATURAL AND MAN-MADE OBSTACLES GOVERNED ALIGNMENT: Beginning at the Charles River, the new river span was to be built with a vertical clearance of 50 feet above the river. Early studies had the dual-deck High Bridge with a 90-foot clearance over the Charles River, but this would have left steeper than desired profiles on both expressway approaches.

Moving south of the High Bridge, the limited distance between the bridge piers and the 13-story Industrial Building left little room for connecting ramps to the newly opened Storrow Drive. Engineers devised a tightly-designed, dual-deck "Y"-interchange between the Central Artery and Storrow Drive. At the southern end of the interchange, the Central Artery transitions from a dual-deck bridge approach to a single-deck viaduct over local streets.

As the single-deck viaduct approaches Haymarket Square, engineers faced another challenge: the entrance to the single-tube Sumner Tunnel (which was joined by its companion - the Callahan Tunnel - in 1961) was located at Haymarket Square. The potential conflict of traffic flows between the tunnel, the expressway and local streets was the subject of 24 different studies. A compromise plan was developed that had four expressway ramps emanating from a large rotary, into which the tunnel approach and local streets also were to feed.

From Haymarket Square south to Fort Hill Square, a four-lane boulevard (the original "Surface Artery") was built underneath the elevated expressway. The high cost of condemnation and acquisition precluded the construction of parallel service roads.BUILDING BOSTON'S OTHER "BIG GREEN MONSTER": An impending sense of doom cast its pall from the North End south to Chinatown as the state began to condemn properties for the Central Artery in 1950. Certainly, there were a number of holdouts owing to the slow pace of demolition, but even they began to lose hope once work began on the High Bridge, which was to carry the six-lane highway across the Charles River. Demolition began in the North End during the fall of 1951, reaching Fort Hill Square two years later. While massive in scale - entire city blocks disappeared - some of the demolition work was so precise that some of it had to be done manually.

During the first months of 1953, soon after demolition began in the North End and the right-of-way was cleared of rubble, workers began driving the concrete piles that supported the steel columns that would hold up the elevated roadway. They also bulldozed thousands of tons of earth for the highway's on- and off-ramps. Some utility and sewer lines had to be moved in preparation for this work, but there was no wholesale relaying of underground lines until the underground Central Artery work began during the early 1990's.

As 1953 wound down and 1954 began, the steel viaduct of the Central Artery started to rise some 30 to 50 feet above the streets of Boston. All of the shop-painted orange viaduct sections were numbered to simplify the installation process. Metal guiderails and light standards were installed during the erection phase to expedite construction. The viaduct was painted in its familiar green color only when all of the viaduct section was completed. All of the steelwork was subcontracted to both a fabricator (the Mount Vernon Bridge Company) and a field erector (the New England Erecting Company). Before the concrete was poured, sewer, steam and electrical lines were installed. The steam pipes were intended to help deice the roadway in the event of a snowstorm or ice storm.

VIADUCT STOPPED, REPLACED BY TUNNEL: The first section of the elevated Central Artery from the High Bridge south to Haymarket square opened to traffic in October 1954, and several months later, a second section opened to traffic south to Fort Hill Square. However, it became quickly clear that the elevated highway not only severed the city from its historic waterfront, but cast a mean shadow over local streets. Far from handling traffic efficiently, the elevated highway and connecting roads often impeded traffic flow.Determined not to repeat the mistakes made along the elevated section, the Massachusetts Department of Public Works (MassDPW) decided to build a six-lane tunnel from Fort Hill Square (Congress Street) south to Chinatown (Kneeland Street). The 2,400-foot-long "South Station Tunnel" (or "Dewey Square Tunnel") was the widest vehicular tunnel in the world when it was built between 1956 and 1959. Unlike the wholesale demolition of buildings further north along the route, less than 10 percent of the total floor area of buildings along the route - which were used by active garment and leather trades - was taken for the tunnel.

The South Station Tunnel was a "cut-and-cover" tunnel built within a steel frame below street level. To build the transition from elevated highway to tunnel, workers dug a 200-foot-wide trench to accommodate the roadway. Before the roadway could be installed, workers had to rebuild the roof of the MBTA Red Line subway tunnel that passed underneath to ensure that the bottom of the roadway tunnel was flush with the top of the subway tunnel. They also had to relocate the Dewey Square subway station closer to South Station. The steel framework was erected only when the tunnel's two 40-foot-wide roadways were in place.

Once the framework was completed, concrete and dirt fill were poured along the walls, and a concrete slab was placed atop the framework. Continuous lighting and tiling were installed, along with an exhaust ventilation system and a television monitoring system (one of the first of its kind).

Above ground, a wide boulevard ("Surface Road") was built atop the tunnel to handle local traffic. According to a 1953 engineering report, the six-lane boulevard was to be "the finest street in downtown Boston."

LEFT: This 1956 photo shows the elevated Central Artery looking south from Haymarket Square. RIGHT: This photo taken the same year shows the elevated expressway descending into the South Street Tunnel. The rest of the expressway was completed in 1959. (Photos from Massachusetts State Transportation Library Archives and Massachusetts Turnpike Authority.)

THE CENTRAL ARTERY OPENS FOR BUSINESS: After more than eight years of construction, the Central Artery and the Southeast Expressway were joined together on June 30, 1959, when both expressways finally opened to traffic. During its first day of operation, some 60,000 vehicles - about two-thirds the 1970 design capacity - used the new six-lane Central Artery. It did not take long for monumental traffic snarls to hit the new highway. The Central Artery recorded its first traffic jam only three months after completion, and by 1960 - just one year after completion - average daily usage already had exceeded its design capacity of 90,000 vehicles per day (AADT).

TAKING THE LOAD FROM ROADS LEFT UNBUILT: The Central Artery was built as part of a network of radial and circumferential routes that were to serve urbanized areas of Boston. Just to the west, a complementary route - the Inner Belt (I-695) was to bypass downtown, but with this route and three other radial expressways completely canceled by 1972, the Central Artery was carrying more than its fair share of traffic.